Thyristor choppers have been in use as means for controlling the dc load of a dc machine such as a dc motor. Such thyristor chopper consists essentially of a main control thyristor and a commutation circuit for turning this thyristor off. Heretofore a variety of commutation circuits have been used in practical applications, consisting essentially of a commutation capacitor, a commutation reactor, and an auxiliary commutation thyristor. In this commutation circuit, the load current is diverted to the commutation capacitor for a given period of time to turn off the main control thyristor. To this effect, therefore, the commutation capacitor and the commutation reactor must have a sufficient capacity so that a current not less than the load current can flow for a given period of time. This, on the other hand, has necessitated increase in the size of the constituent elements and lowered the chopper efficiency. Furthermore, this type of chopper cannot be used at high frequencies because the commutation must take place at the charge-discharge cycle of the commutation capacitor.
Prior art techniques have provided current feedback type transistor choppers in which part of the load current is supplied to the base of the main control transistor through a current transformer to turn on this transistor, and the current transformer is reset by the load commutation current while the main control transistor is in the off state. In this type of chopper, although the operating efficiency is fairly high since the base current of the main control transistor is limited to a value corresponding to the load current, the transformer is disabled when it is saturated, with the result that the transistor becomes nonconducting, because the transistor depends for its base current upon the transformer. In other words, the time for which the transistor is on is limited due to saturation in the transformer. This has hampered prior art techniques from realizing a chopper device operable with full commutation efficiency. The present invention obviates the foregoing prior art drawbacks.